In combustors which use coal as fuel, at times there will be a loss of efficiency in burning the coal which results in unburned carbon particles being emitted in the exhaust. The exhaust from such a combustor will contain fine solid particles of ashes, dust, and carbonaceous material carried out from the burning fuel by the exhaust draft, and is referred to in the field as fly ash. The presence of unburned carbon particles in fly ash is an indicator that there is a loss of efficiency in the system. Detecting such particles is important in calculating the combustion efficiency in coal-fired combustors such as boilers.
Present methods which are used in measuring unburned carbon particles in fly ash are tedious and time consuming. Carbon which is emitted from combustors can be classified as either grit or soot. Grit is defined as the material which is retained on a 200-mesh USA sieve and represents unburned char. Soot is composed of submicron particles less than 0.1 .mu.m in diameter, is formed by gas phase processes. The mass of soot which leaves a combustor is too small to effect combustion efficiency. Currently, unburned carbon in fly ash is measured by sampling flue gas from the exhaust of a combustor and using a particulate filter to remove fly ash from the flue. The fly ash is then dried and weighed before being placed in a furnace for several hours in order to burn off the carbon. The fly ash is then reweighed to find the percent of carbon. This can involve several hours and rarely yields a large enough sample through filtering to investigate a transient phenomenon in the combustor. The accuracy of the technique is also limited by problems associated with isokinetic sampling and filtration techniques.
This invention relates to an improved method of measuring the unburned carbon content in fly ash. It involves the application of photoacoustic absorption spectroscopy as a method for on-line monitoring of unburned carbon in fly ash. Photoacoustic absorption spectroscopy (PAS) is based on the periodic heating of a gas when amplitude-modulated radiation is absorbed by the gas or by particles suspended in the gas. This technique has been used, for example, to measure mass concentrations of soot in the exhaust of diesel engines, such as that discussed in Japar, S. M., Moore, J., and Killinger, D. K., "The Use of Photoacoustic Spectroscopy to Characterize and Monitor Soot in Combustion Processes.", Combustion Science Technology, Vol. 24, pp. 215-219 (1981). However, this process has not been applied to particles as large as unburned carbon in fly ash. Instead, it is applied towards soot and similar material which has a diameter of about 0.1 microns or less.
Two important features of PAS make it especially suitable for detection of carbon in flue gas. The first of these is the technique's ability to detect very weak absorptions. Secondly, the PAS signal is unaffected by light scattering in a particulate-laden gas flow. Instead, the acoustical signal arises only from light absorption.
Thus, it is a primary object of this invention to provide for an improved method and apparatus of measuring the unburned carbon content of exhaust from coal-fired combustors.
A further object of the invention is to provide for a method of detecting unburned carbon in combustion exhaust which employs photoacoustic absorption spectroscopy.
Yet another object of the invention is to provide for a sensitive method and apparatus of measuring unburned carbon content in combustion exhaust.
A still further object of the invention is to provide for a method of measuring unburned carbon content in combustor exhaust using photoacoustic absorption spectroscopy in which the signal is unaffected by light scattering in a particulate-laden gas flow.
Yet another object of the invention is to provide for an accurate and time efficient method and apparatus of measuring unburned carbon content in combustor exhaust.
Further objects of the invention will become apparent with the following disclosure.